Solar cells produced from high Ohmic wafers and halogen containing paste

ABSTRACT

In general, the present invention relates to electro-conductive pastes with halogen containing compounds as additives and solar cells with high Ohmic sheet resistance, preferably photovoltaic solar cells. More specifically, the present invention relates to solar cell precursors, processes for preparation of solar cells, solar cells and solar modules. The present invention relates to a solar cell precursor at least comprising as precursor parts:
         i) a wafer with sheet resistance of at least 80 Ohm/sq.;   ii) an electro-conductive paste at least comprising:
           a) metallic particles;   b) a glass frit;   c) an organic vehicle; and   d) a halogen containing compound   applied to the wafer.

This application claims the benefit of U.S. Provisional Application No.61/713,423, filed Oct. 12, 2012, European Patent Application No.12007106.3, filed on Oct. 12, 2012, and European Patent Application No.13002639.6, filed May 21, 2013, each which is incorporated by referencein its entirety.

FIELD OF THE INVENTION

In general, the present invention relates to electro-conductive pasteswith halogen containing compounds as additives and solar cells with highOhmic sheet resistance, preferably photovoltaic solar cells. Morespecifically, the present invention relates to solar cell precursors,processes for preparation of solar cells, solar cells and solar modules.

BACKGROUND OF THE INVENTION

Solar cells are devices that convert the energy of light intoelectricity using the photovoltaic effect. Solar power is an attractivegreen energy source because it is sustainable and produces onlynon-polluting by-products. Accordingly, a great deal of research iscurrently being devoted to developing solar cells with enhancedefficiency while continuously lowering material and manufacturing costs.When light hits a solar cell, a fraction of the incident light isreflected by the surface and the remainder transmitted into the solarcell. The transmitted photons are absorbed by the solar cell, which isusually made of a semiconducting material, such as silicon which isoften doped appropriately. The absorbed photon energy excites electronsof the semiconducting material, generating electron-hole pairs. Theseelectron-hole pairs are then separated by p-n junctions and collected byconductive electrodes on the solar cell surfaces. FIG. 1 shows a minimalconstruction for a simple solar cell.

Solar cells are very commonly based on silicon, often in the form of aSi wafer. Here, a p-n junction is commonly prepared either by providingan n-type doped Si substrate and applying a p-type doped layer to oneface or by providing a p-type doped Si substrate and applying an n-typedoped layer to one face to give in both cases a so called p-n junction.The face with the applied layer of dopant generally acts as the frontface of the cell, the opposite side of the Si with the original dopantacting as the back face. Both n-type and p-type solar cells are possibleand have been exploited industrially. Cells designed to harness lightincident on both faces are also possible, but their use has been lessextensively harnessed.

In order to allow incident light on the front face of the solar cell toenter and be absorbed, the front electrode is commonly arranged in twosets of perpendicular lines known as “fingers” and “bus bars”respectively. The fingers form an electrical contact with the front faceand bus bars link these fingers to allow charge to be drawn offeffectively to the external circuit. It is common for this arrangementof fingers and bus bars to be applied in the form of anelectro-conductive paste which is fired to give solid electrode bodies.A back electrode is also often applied in the form of anelectro-conductive paste which is then fired to give a solid electrodebody. A typical electro-conductive paste contains metallic particles,glass frit, and an organic vehicle.

Recently, it has been found that solar cells based on wafers with highsheet resistance, often with a sheet resistance above 80 Ohm/sq., socalled high Ohmic wafers, have the potential for increased cellperformance. However, disadvantages exist in connection with the use ofhigh Ohmic wafers for producing solar cells, particularly in the form ofhigh contact resistance of the contact between such wafers andelectrodes.

There is thus a need in the state of the art for improvements to theapproach to producing solar cells from high Ohmic wafers.

SUMMARY OF THE INVENTION

The present invention is generally based on the object of overcoming atleast one of the problems encountered in the state of the art inrelation to solar cells, in particular in relation to those solar cellsbased on wafers with a high sheet resistance and those with a low dopantlevel on the front face, commonly referred to as high Ohmic wafers.

More specifically, the present invention is further based on the objectof providing solar cells with improved performance, in particularreduced contact resistance between electrodes and wafers in particularbetween electrodes and such high Ohmic wafers.

A further object of the present invention is to provide processes forpreparing solar cells, particularly solar cells based on wafers of highOhmic resistance and wherein the contact resistance between electrodesand wafer is.

A contribution to achieving at least one of the above described objectsis made by the subject matter of the category forming claims of thepresent invention. A further contribution is made by the subject matterof the dependent claims of the present invention which representspecific embodiments of the present invention.

DETAILED DESCRIPTION

A contribution to achieving at least one of the above described objectsis made by a solar cell precursor comprising as precursor parts atleast:

-   -   i) a wafer with a sheet resistance of at least 80 Ohm/sq.;    -   ii) an electro-conductive paste at least comprising:        -   a) metallic particles;        -   b) a glass frit;        -   c) an organic vehicle;        -   d) a halogen containing compound; and        -   e) an additive        -   on the wafer.

In one embodiment of the present invention, the electro-conductive pasteis on the front face of the wafer.

Wafer

Preferred wafers according to the present invention are regions amongother regions of the solar cell capable of absorbing light with highefficiency to yield electron-hole pairs and separating holes andelectrons across a boundary with high efficiency, preferably across a socalled p-n junction boundary. Preferred wafers according to the presentinvention are those comprising a single body made up of a front dopedlayer and a back doped layer.

It is preferred for that wafer to consist of appropriately dopedtetravalent elements, binary compounds, tertiary compounds or alloys.Preferred tetravalent elements in this context are Si, Ge or Sn,preferably Si. Preferred binary compounds are combinations of two ormore tetravalent elements, binary compounds of a group III element witha group V element, binary compounds of a group II element with a groupVI element or binary compounds of a group IV element with a group VIelement. Preferred combinations of tetravalent elements are combinationsof two or more elements selected from Si, Ge, Sn or C, preferably SiC.The preferred binary compounds of a group III element with a group Velement is GaAs. It is most preferred according to the present inventionfor the wafer to be based on Si. Si, as the most preferred material forthe wafer, is referred to explicitly throughout the rest of thisapplication. Sections of the following text in which Si is explicitlymentioned also apply for the other wafer compositions described above.

Where the front doped layer and back doped layer of the wafer meet isthe p-n junction boundary. In an n-type solar cell, the back doped layeris doped with electron donating n-type dopant and the front doped layeris doped with electron accepting or hole donating p-type dopant. In ap-type solar cell, the back doped layer is doped with p-type dopant andthe front doped layer is doped with n-type dopant. It is preferredaccording to the present invention to prepare a wafer with a p-njunction boundary by first providing a doped Si substrate and thenapplying a doped layer of the opposite type to one face of thatsubstrate.

Doped Si substrates are well known to the person skilled in the art. Thedoped Si substrate can be prepared in any way known to the personskilled in the art and which he considers to be suitable in the contextof the present invention. Preferred sources of Si substrates accordingto the present invention are mono-crystalline Si, multi-crystalline Si,amorphous Si and upgraded metallurgical Si, mono-crystalline Si ormulti-crystalline Si being most preferred. Doping to form the doped Sisubstrate can be carried out simultaneously by adding dopant during thepreparation of the Si substrate or can be carried out in a subsequentstep. Doping subsequent to the preparation of the Si substrate can becarried out for example by gas diffusion epitaxy. Doped Si substratesare also readily commercially available. According to the presentinvention it is one option for the initial doping of the Si substrate tobe carried out simultaneously to its formation by adding dopant to theSi mix. According to the present invention it is one option for theapplication of the front doped layer and the highly doped back layer, ifpresent, to be carried out by gas-phase epitaxy. This gas phase epitaxyis preferably carried out at a temperature in a range from 500° C. to900° C., more preferably in a range from 600° C. to 800° C. and mostpreferably in a range from 650° C. to 750° C. at a pressure in a rangefrom 2 kPa and 100 kPa, preferably in a range from 10 to 80 kPa, mostpreferably in a range from 30 to 70 kPa.

It is known to the person skilled in the art that Si substrates canexhibit a number of shapes, surface textures and sizes. The shape can beone of a number of different shapes including cuboid, disc, wafer andirregular polyhedron amongst others. The preferred shape according tothe present invention is wafer shaped where that wafer is a cuboid withtwo dimensions which are similar, preferably equal and a third dimensionwhich is significantly less than the other two dimensions. Significantlyless in this context is preferably at least a factor of 100 smaller.

A variety of surface types are known to the person skilled in the art.According to the present invention Si substrates with rough surfaces arepreferred. One way to assess the roughness of the substrate is toevaluate the surface roughness parameter for a sub-surface of thesubstrate which is small in comparison to the total surface area of thesubstrate, preferably less than one hundredth of the total surface area,and which is essentially planar. The value of the surface roughnessparameter is given by the ratio of the area of the subsurface to thearea of a theoretical surface formed by projecting that subsurface ontothe flat plane best fitted to the subsurface by minimising mean squaredisplacement. A higher value of the surface roughness parameterindicates a rougher, more irregular surface and a lower value of thesurface roughness parameter indicates a smoother, more even surface.According to the present invention, the surface roughness of the Sisubstrate is preferably modified so as to produce an optimum balancebetween a number of factors including but not limited to lightabsorption and adhesion of fingers to the surface.

The two dimensions with larger scale of the Si substrate can be variedto suit the application required of the resultant solar cell. It ispreferred according to the present invention for the thickness of the Siwafer to lie below 0.5 mm more preferably below 0.3 mm and mostpreferably below 0.2 mm Some wafers have a minimum size of 0.01 mm ormore.

It is preferred according to the present invention for the front dopedlayer to be thin in comparison to the back doped layer. It is preferredaccording to the present invention for the front doped layer to have athickness lying in a range from 0.1 to 10 μm, preferably in a range from0.1 to 5 μm and most preferably in a range from 0.1 to 2 μm.

A highly doped layer can be applied to the back face of the Si substratebetween the back doped layer and any further layers. Such a highly dopedlayer is of the same doping type as the back doped layer and such alayer is commonly denoted with a + (n⁺-type layers are applied to n-typeback doped layers and p⁺-type layers are applied to p-type back dopedlayers). This highly doped back layer serves to assist metallisation andimprove electro-conductive properties at the substrate/electrodeinterface area. It is preferred according to the present invention forthe highly doped back layer, if present, to have a thickness in a rangefrom 1 to 100 μm, preferably in a range from 1 to 50 μm and mostpreferably in a range from 1 to 15 μm.

Dopants

Preferred dopants are those which, when added to the Si wafer, form ap-n junction boundary by introducing electrons or holes into the bandstructure. It is preferred according to the present invention that theidentity and concentration of these dopants is specifically selected soas to tune the band structure profile of the p-n junction and set thelight absorption and conductivity profiles as required. Preferred p-typedopants according to the present invention are those which add holes tothe Si wafer band structure. They are well known to the person skilledin the art. All dopants known to the person skilled in the art and whichhe considers to be suitable in the context of the present invention canbe employed as p-type dopant. Preferred p-type dopants according to thepresent invention are trivalent elements, particularly those of group 13of the periodic table. Preferred group 13 elements of the periodic tablein this context include but are not limited to B, Al, Ga, In, Tl or acombination of at least two thereof, wherein B is particularlypreferred.

Preferred n-type dopants according to the present invention are thosewhich add electrons to the Si wafer band structure. They are well knownto the person skilled in the art. All dopants known to the personskilled in the art and which he considers to be suitable in the contextof the present invention can be employed as n-type dopant. Preferredn-type dopants according to the present invention are elements of group15 of the periodic table. Preferred group 15 elements of the periodictable in this context include N, P, As, Sb, Bi or a combination of atleast two thereof, wherein P is particularly preferred.

As described above, the various doping levels of the p-n junction can bevaried so as to tune the desired properties of the resulting solar cell.It is preferred according to the present invention for the wafer to havea sheet resistance of at least 80 Ohm/sq., more preferably at least 90Ohm/sq. and most preferably at least 100 Ohm/sq. In some cases, amaximum value of 200 Ohm/sq. is observed for the sheet resistance ofhigh Ohmic wafers.

According to the present invention, it is preferred for the back dopedlayer to be lightly doped, preferably with a dopant concentration in arange from 1×10¹³ to 1×10¹⁸ cm⁻³, preferably in a range from 1×10¹⁴ to1×10¹⁷ cm⁻³, most preferably in a range from 5×10¹⁵ to 5×10¹⁶ cm⁻³. Somecommercial products have a back doped layer with a dopant concentrationof about 1×10¹⁶.

It is preferred according to the present invention for the highly dopedback layer (if one is present) to be highly doped, preferably with aconcentration in a range from 1×10¹⁷ to 5×10²¹ cm⁻³, more preferably ina range from 5×10¹⁷ to 5×10²⁰ cm⁻³, and most preferably in a range from1×10¹⁸ to 1×10¹⁹ cm⁻³.

Electro-Conductive Paste

Preferred electro-conductive pastes according to the present inventionare pastes which can be applied to a surface and which, on firing, formsolid electrode bodies in electrical contact with that surface.Preferred electro-conductive pastes in the context of the presentinvention are those which comprise as paste components:

-   -   i) metallic particles, preferably at least 50 wt. %, more        preferably at least 70 wt. % and most preferably at least 80 wt.        %;    -   ii) glass frit, preferably in a range of 0.1 to 15 wt. %, more        preferably in a range of 0.1 to 10 wt. % and most preferably in        a range of 0.1 to 5 wt. %;    -   iii) organic vehicle, preferably in a range of 5 to 40 wt. %,        more preferably in a range of 5 to 30 wt. % and most preferably        in a range of 5 to 15 wt. %;    -   iv) a halogen containing compound, preferably with a        concentration in the ranges given below; and    -   v) additives, preferably in a range from 0 to 15 wt. %, more        preferably in a range of 0 to 10 wt. % and most preferably in a        range of 0.1 to 5 wt. %,        wherein the wt. % are each based on the total weight of the        electro-conductive paste.

In order to facilitate printability of the electro-conductive paste, itis preferred according to the present invention that the viscosity ofthe electro-conductive paste lie in a range from 10-30 Pa*s, preferablyin a range from 12-25 Pa*s and most preferably in a range from 15-22Pa*s.

In one embodiment of the solar cell precursor according to the presentinvention, the electro-conductive paste is on the front face of thewafer. In further embodiments, the electro conductive paste is on theback face of the wafer or even on both faces and/or in a holepenetrating the wafer. Such holes are often called via holes and arecommonly used in so called metal wrap through designs which aredescribed in WO 2012/026812 A1 and WO 2012/026806 A1.

Halogen Containing Compound

Preferred halogen containing compounds in the context of the presentinvention are those which contain halogen atoms or ions, preferably oneor more selected from Cl, Br or I atoms or ions, most preferably Clatoms or ions. The halogen atoms or ions used herein include alloxidation states. It is preferred that the halogen containing compoundis an inorganic compound, preferably an inorganic salt.

Preferred halogen compounds in this connection are halide salts,oxyhalonium salts and combinations thereof. Preferred halide ions areCl⁻, Br⁻, I⁻, or combinations thereof, preferably Cl⁻. Preferredoxyhalonium ions are ClO₂ ⁻, ClO₃ ⁻, ClO₄ ⁻, BrO₂ ⁻, BrO₃ ⁻, BrO₄ ⁻, IO₂⁻, IO₃ ⁻, IO₄ ⁻, IO₆ ⁵⁻, IO₅ ³⁻, I₂O₉ ⁴⁻, or I₂O₁₁ ⁸⁻, or a combinationof at least two thereof, preferably ClO₃ ⁻. Preferred counter ions inthis context are metal ions, preferably transition metal orpost-transition metal ions, or a combination of at least two thereof.Preferred counter-ions in this context are Pb²⁺, Zn²⁺, Li⁺, Ni²⁺, Al³⁺,Rh¹⁺, Ag⁺, Ba²⁺, or K⁺ or combinations of at least two thereof,preferably Ag⁺, Ba²⁺, or K⁺ or combinations of at least two thereof.Preferred transition metal ions are Ag⁺ or Zn²⁺, or combinations ofboth. Preferred post transition metal ions are Pb²⁺, In³⁺, or acombination of both. The most preferred counter-ion in this context isAg⁺. Examples of oxyhalonium salts and decomposition data can be foundin J. Phys. Chem. Ref. Data, Vol. 3 no. 2, 1974.

In one embodiment of the present invention, the halogen containingcompound comprises one or more selected from Cl, Br, or I atoms, or therespective ions thereof. In a further preferred embodiment of thepresent invention, the halogen containing compound comprises Cl atoms orions. In a further preferred embodiment of the present invention, thehalogen containing compound comprises a chloride containing salt. In afurther preferred embodiment of the present invention, the halogencontaining compound comprises a one or more of PbCl₂, AgCl, InCl₃,ZnCl₂. In a further preferred embodiment of the present invention, thehalogen containing compound comprises AgCl.

In a further embodiment of the present invention, the halogen containingcompound comprises a palladium halide. Preferred palladium halides inthis context comprise Cl, Br, or I atoms/ions, preferably Cl atoms/ions.Preferred palladium halides are those which comprise at least a furthermetal in addition to Pd. Preferred further metals in this connection arealkali metals and alkali earth metals, preferably alkali metals.Preferred alkali metals are Li, Na, K, Rb, or Cs, preferably Li, Na orK, most preferably Na. In one aspect of this embodiment of the presentinvention, the halogen containing compound comprises Na₂PdCl₄.

In the case where the halogen containing compound contains halogen inthe form of simple halide ions, it is preferred for this compound to bepresent at the maximum firing temperature in the liquid state and for itnot to have decomposed by the maximum firing temperature. Accordingly,it is preferred for such compounds which contain halogen in the form ofsimple halide ions to have a melting point below the maximum firingtemperature, a boiling point above the maximum firing temperature and adecomposition temperature above the maximum firing temperature. In thecase where the halogen containing compound does not contain halogen inthe form of simple halide ions, it is preferred for this compound todecompose at a temperature below the maximum firing temperature and forsimple halide ions to be present in one of the products of thatdecomposition, preferably a product which is present in the liquid stateat the maximum firing temperature. Accordingly it is preferred for suchcompounds which do not contain halogen in the form of simple halide ionsto have a decomposition temperature below the maximum firing temperatureand a decomposition product containing halogen in the form of simplehalide ions with a melting point below the maximum firing temperatureand a boiling point above the maximum firing temperature.

In one embodiment of the present invention, the halogen containingcompound has a decomposition temperature below the highest peak firingtemperature, preferably below 900° C., more preferably below 700° C.,most preferably below 600° C. In one aspect of this embodiment, thedecomposition products comprise a compound which contains halide ionsand which has a boiling point above 700° C., preferably above 800° C.,more preferably above 900° C. In a further aspect of this embodiment,the decomposition products comprise a compound which contains halideions and which has a melting point in a range from 180 to 800° C., morepreferably in a range from 200 to 800° C. and most preferably in a rangefrom 250 to 750° C.

In a further embodiment of the present invention, the halogen containingcompound has a boiling temperature or decomposition temperature or bothabove 500° C., preferably above 600° C., more preferably above 700° C.

In one embodiment of the present invention, the halogen containingcompound is preferably present in the electro-conductive paste in aconcentration in a range from 0.1 to 40 mmol/kg more preferably in arange from 1 to 30 mmol/kg, and most preferably in a range from 10 to 20mmol/kg, in each case based on the number of halogen atoms/ions and onthe total weight of the paste.

Metallic Particles

Preferred metallic particles in the context of the present invention arethose which exhibit metallic conductivity or which yield a substancewhich exhibits metallic conductivity on firing. Metallic particlespresent in the electro-conductive paste gives metallic conductivity tothe solid electrode which is formed when the electro-conductive paste issintered on firing. Metallic particles which favour effective sinteringand yield electrodes with high conductivity and low contact resistanceare preferred. Metallic particles are well known to the person skilledin the art. All metallic particles known to the person skilled in theart and which he considers suitable in the context of the presentinvention can be employed as the metallic particles in theelectro-conductive paste. Preferred metallic particles according to thepresent invention are metals, alloys, mixtures of at least two metals,mixtures of at least two alloys or mixtures of at least one metal withat least one alloy.

Preferred metals which can be employed as metallic particles accordingto the present invention are Ag, Cu, Al, Zn, Pd, Ni or Pb and mixturesof at least two thereof, preferably Ag. Preferred alloys which can beemployed as metallic particles according to the present invention arealloys containing at least one metal selected from the list of Ag, Cu,Al, Zn, Ni, W, Pb and Pd or mixtures or two or more of those alloys.

In one embodiment according to the present invention, the metallicparticles comprise a metal or alloy coated with one or more furtherdifferent metals or alloys, for example copper coated with silver.

In one embodiment according to the present invention, the metallicparticles comprise Ag. In another embodiment according to the presentinvention, the metallic particles comprise a mixture of Ag with Al.

As additional constituents of the metallic particles, further to abovementioned constituents, those constituents which contribute to morefavourable sintering properties, electrical contact, adhesion andelectrical conductivity of the formed electrodes are preferred accordingto the present invention. All additional constituents known to theperson skilled in the art and which he considers to be suitable in thecontext of the present invention can be employed in the metallicparticles. Those additional substituents which represent complementarydopants for the face to which the electro-conductive paste is to beapplied are preferred according to the present invention. When formingan electrode interfacing with an n-type doped Si layer, additivescapable of acting as n-type dopants in Si are preferred. Preferredn-type dopants in this context are group 15 elements or compounds whichyield such elements on firing. Preferred group 15 elements in thiscontext according to the present invention are P and Bi. When forming anelectrode interfacing with a p-type doped Si layer, additives capable ofacting as p-type dopants in Si are preferred. Preferred p-type dopantsare group 13 elements or compounds which yield such elements on firing.Preferred group 13 elements in this context according to the presentinvention are B and Al.

It is well known to the person skilled in the art that metallicparticles can exhibit a variety of shapes, surfaces, sizes, surface areato volume ratios, oxygen content and oxide layers. A large number ofshapes are known to the person skilled in the art. Some examples arespherical, angular, elongated (rod or needle like) and flat (sheetlike). Metallic particles may also be present as a combination ofparticles of different shapes. Metallic particles with a shape, orcombination of shapes, which favours advantageous sintering, electricalcontact, adhesion and electrical conductivity of the produced electrodeare preferred according to the present invention. One way tocharacterise such shapes without considering surface nature is throughthe parameters length, width and thickness. In the context of thepresent invention the length of a particle is given by the length of thelongest spatial displacement vector, both endpoints of which arecontained within the particle. The width of a particle is given by thelength of the longest spatial displacement vector perpendicular to thelength vector defined above both endpoints of which are contained withinthe particle. The thickness of a particle is given by the length of thelongest spatial displacement vector perpendicular to both the lengthvector and the width vector, both defined above, both endpoints of whichare contained within the particle. In one embodiment according to thepresent invention, metallic particles with shapes as uniform as possibleare preferred i.e. shapes in which the ratios relating the length, thewidth and the thickness are as close as possible to 1, preferably allratios lying in a range from 0.7 to 1.5, more preferably in a range from0.8 to 1.3 and most preferably in a range from 0.9 to 1.2. Examples ofpreferred shapes for the metallic particles in this embodiment aretherefore spheres and cubes, or combinations thereof, or combinations ofone or more thereof with other shapes. In another embodiment accordingto the present invention, metallic particles are preferred which have ashape of low uniformity, preferably with at least one of the ratiosrelating the dimensions of length, width and thickness being above 1.5,more preferably above 3 and most preferably above 5. Preferred shapesaccording to this embodiment are flake shaped, rod or needle shaped, ora combination of flake shaped, rod or needle shaped with other shapes.

A variety of surface types are known to the person skilled in the art.Surface types which favour effective sintering and yield advantageouselectrical contact and conductivity of produced electrodes are favouredfor the surface type of the metallic particles according to the presentinvention.

Another way to characterise the shape and surface of a metallic particleis by its surface area to volume ratio. The lowest value for the surfacearea to volume ratio of a particle is embodied by a sphere with a smoothsurface. The less uniform and uneven a shape is, the higher its surfacearea to volume ratio will be. In one embodiment according to the presentinvention, metallic particles with a high surface area to volume ratioare preferred, preferably in a range from 1.0×10⁷ to 1.0×10⁹ m⁻¹, morepreferably in a range from 5.0×10⁷ to 5.0×10⁸ m⁻¹ and most preferably ina range from 1.0×10⁸ to 5.0×10⁸ m⁻¹. In another embodiment according tothe present invention, metallic particles with a low surface area tovolume ratio are preferred, preferably in a range from 6×10⁵ to 8.0×10⁶m⁻¹, more preferably in a range from 1.0×10⁶ to 6.0×10⁶ m⁻¹ and mostpreferably in a range from 2.0×10⁶ to 4.0×10⁶ m⁻¹.

The particles diameter d₅₀ and the associated values d₁₀ and d₉₀ arecharacteristics of particles well known to the person skilled in theart. It is preferred according to the present invention that the averageparticle diameter d₅₀ of the metallic particles lie in a range from 0.5to 10 μm, more preferably in a range from 1 to 10 μm and most preferablyin a range from 1 to 5 μm. The determination of the particles diameterd₅₀ is well known to a person skilled in the art.

The metallic particles may be present with a surface coating. Any suchcoating known to the person skilled in the art and which he considers tobe suitable in the context of the present invention can be employed onthe metallic particles. Preferred coatings according to the presentinvention are those coatings which promote improved printing, sinteringand etching characteristics of the electro-conductive paste. If such acoating is present, it is preferred according to the present inventionfor that coating to correspond to no more than 10 wt. %, preferably nomore than 8 wt. %, most preferably no more than 5 wt. %, in each casebased on the total weight of the metallic particles.

In one embodiment according to the present invention, the metallicparticles are present as a proportion of the electro-conductive pastemore than 50 wt. %, preferably more than 70 wt. %, most preferably morethan 80 wt. %.

Glass Frit

Preferred glass frits in the context of the present invention arepowders of amorphous or partially crystalline solids which exhibit aglass transition. The glass transition temperature T_(g) is to thetemperature where an amorphous substance transfers from a rigid solid toa partially mobile undercooled melt upon heating. Methods for thedetermination of the glass transition temperature are well known to theperson skilled in the art. Glass frit is present in theelectroconductive paste according to the present invention in order tobring about etching and sintering. Effective etching is required to etchthrough any additional layers which may have been applied to the Siwafer and thus lie between the front doped layer and the appliedelectro-conductive paste as well as to etch into the Si wafer to anappropriate extent. Appropriate etching of the Si wafer means deepenough to bring about good electrical contact between the electrode andthe front doped layer and thus lead to a low contact resistance but notas deep as to interfer with the p-n junction boundary. The etching andsintering brought about by the glass frit occurs above the glasstransition temperature of the glass frit and the glass transitiontemperature must lie below the desired peak firing temperature. Glassfrits are well known to the person skilled in the art. All glass fritsknown to the person skilled in the art and which he considers suitablein the context of the present invention can be employed as the glassfrit in the electro-conductive paste.

In the context of the present invention, the glass frit present in theelectro-conductive paste preferably comprises elements, oxides,compounds which generate oxides on heating, other compounds, or mixturesthereof. Preferred elements in this context are Si, B, Al, Bi, Li, Na,Mg, Pb, Zn, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, Ba and Cr ormixtures of two or more from this list. Preferred oxides which can becomprised by the glass frit in the context of the present invention arealkali metal oxides, alkali earth metal oxides, rare earth oxides, groupV and group VI oxides, other oxides, or combinations thereof. Preferredalkali metal oxides in this context are sodium oxide, lithium oxide,potassium oxide, rubidium oxides, caesium oxides or combinationsthereof. Preferred alkali earth metal oxides in this context areberyllium oxide, magnesium oxide, calcium oxide, strontium oxide, bariumoxide, or combinations thereof. Preferred group V oxides in this contextare phosphorous oxides, such as P₂O₅, bismuth oxides, such as Bi₂O₃, orcombinations thereof. Preferred group VI oxides in this context aretellurium oxides, such as TeO₂, or TeO₃, selenium oxides, such as SeO₂,or combinations thereof. Preferred rare earth oxides are cerium oxides,such as CeO₂ and lanthanum oxides, such as La₂O₃. Other preferred oxidesin this context are silicon oxides, such as SiO₂, zinc oxides, such asZnO, aluminium oxides, such as Al₂O₃, germanium oxides, such as GeO₂,vanadium oxides, such as V₂O₅, niobium oxides, such as Nb₂O₅, boronoxide, tungsten oxides, such as WO₃, molybdenum oxides, such as MoO₃,and indium oxides, such as In₂O₃, further oxides of those elementslisted above as preferred elements, or combinations thereof. Preferredoxides are also mixed oxides containing at least two of the elementslisted as preferred elemental constituents of the frit glass, or mixedoxides which are formed by heating at least one of the above namedoxides with at least one of the above named metals. Mixtures of at leasttwo of the above-listed oxides and mixed oxides are also preferred inthe context of the present invention.

As mentioned above, the glass frit must have a glass transitiontemperature below the desired firing temperature of theelectro-conductive paste. According to the present invention, preferredglass frits have a glass transition temperature in the range 250° C. to700° C., preferably in the range 300° C. to 600° C. and most preferablyin the range 350° C. to 500° C.

It is well known to the person skilled in the art that glass fritparticles can exhibit a variety of shapes, surface natures, sizes,surface area to volume ratios and coating layers. A large number ofshapes of glass frit particles are known to the person skilled in theart. Some examples are spherical, angular, elongated (rod or needlelike) and flat (sheet like). Glass frit particles may also be present asa combination of particles of different shapes. Glass frit particleswith a shape, or combination of shapes, which favours advantageoussintering, adhesion, electrical contact and electrical conductivity ofthe produced electrode are preferred according to the present invention.

A way to characterise the shape and surface of a particle is by itssurface area to volume ratio. The lowest value for the surface area tovolume ratio of a particle is embodied by a sphere with a smoothsurface. The less uniform and uneven a shape is, the higher its surfacearea to volume ratio will be. In one embodiment according to the presentinvention, glass frit particles with a high surface area to volume ratioare preferred, preferably in a range from 1.0×10⁷ to 1.0×10⁹ m⁻¹, morepreferably in a range from 5.0×10⁷ to 5.0×10⁸ m⁻¹ and most preferably ina range from 1.0×10⁸ to 5.0×10⁸ m⁻¹. In another embodiment according tothe present invention, glass frit particles with a low surface area tovolume ratio are preferred, preferably in a range from 6×10⁵ to 8.0×10⁶m⁻¹, more preferably in a range from 1.0×10⁶ to 6.0×10⁶ m⁻¹ and mostpreferably in a range from 2.0×10⁶ to 4.0×10⁶ m⁻¹.

The average particles diameter d₅₀, and the associated parameters d₁₀and d₉₀ are characteristics of particles well known to the personskilled in the art. It is preferred according to the present inventionthat the average particle diameter d₅₀ of the glass frit lie in a rangefrom 0.5 to 10 μm, more preferably in a range from 1 to 7 μm and mostpreferably in a range from 1 to 5 μm. The determination of the particlesdiameter d₅₀ is well known to a person skilled in the art.

The glass frit particles may be present with a surface coating. Any suchcoating known to the person skilled in the art and which he considers tobe suitable in the context of the present invention can be employed onthe glass frit particles. Preferred coatings according to the presentinvention are those coatings which promote improved printing, sinteringand etching characteristics of the electro-conductive paste. If such acoating is present, it is preferred according to the present inventionfor that coating to correspond to no more than 10 wt. %, preferably nomore than 8 wt. %, most preferably no more than 5 wt. %, in each casebased on the total weight of the glass frit particles.

In one embodiment according to the present invention, the glass frit ispresent as a proportion of the electro-conductive paste less than 7 wt.%, preferably less than 6 wt. %, more preferably less than 5 wt. % andmost preferably less than 4 wt. %. In some cases, glass frit proportionsas low as 0.02 wt. % have been employed in electro-conductive pastes.

Organic Vehicle

Preferred organic vehicles in the context of the present invention aresolutions, emulsions or dispersions based on a one or more solvents,preferably an organic solvent, which ensure that the constituents of theelectro-conductive paste are present in a dissolved, emulsified ordispersed form. Preferred organic vehicles are those which provideoptimal stability of constituents within the electro-conductive pasteand endow the electro-conductive paste with a viscosity allowingeffective line printability. Preferred organic vehicles according to thepresent invention comprise as vehicle components:

-   -   (i) a binder, preferably in a range of 1 to 10 wt. %, more        preferably in a range of 2 to 8 wt. % and most preferably in a        range of 3 to 7 wt. %;    -   (ii) a surfactant, preferably in a range of 0 to 10 wt. %, more        preferably in a range of 0 to 8 wt. % and most preferably in a        range of 0.1 to 6 wt. %;    -   (ii) one or more solvents, the proportion of which is determined        by the proportions of the other constituents in the organic        vehicle;    -   (iv) additives, preferably in range of 0 to 15 wt. %, more        preferably in a range of 0 to 13 wt. % and most preferably in a        range of 5 to 11 wt. %,        wherein the wt. % are each based on the total weight of the        organic vehicle and add up to 100 wt. %. According to the        present invention preferred organic vehicles are those which        allow for the preferred high level of printability of the        electro-conductive paste described above to be achieved.        Binder

Preferred binders in the context of the present invention are thosewhich contribute to the formation of an electro-conductive paste withfavourable stability, printability, viscosity, sintering and etchingproperties. Binders are well known to the person skilled in the art. Allbinders which are known to the person skilled in the art and which heconsiders to be suitable in the context of this invention can beemployed as the binder in the organic vehicle. Preferred bindersaccording to the present invention (which often fall within the categorytermed “resins”) are polymeric binders, monomeric binders, and binderswhich are a combination of polymers and monomers. Polymeric binders canalso be copolymers where at least two different monomeric units arecontained in a single molecule. Preferred polymeric binders are thosewhich carry functional groups in the polymer main chain, those whichcarry functional groups off of the main chain and those which carryfunctional groups both within the main chain and off of the main chain.Preferred polymers carrying functional groups in the main chain are forexample polyesters, substituted polyesters, polycarbonates, substitutedpolycarbonates, polymers which carry cyclic groups in the main chain,poly-sugars, substituted poly-sugars, polyurethanes, substitutedpolyurethanes, polyamides, substituted polyamides, phenolic resins,substituted phenolic resins, copolymers of the monomers of one or moreof the preceding polymers, optionally with other co-monomers, or acombination of at least two thereof. Preferred polymers which carrycyclic groups in the main chain are for example polyvinylbutylate (PVB)and its derivatives and poly-terpineol and its derivatives or mixturesthereof. Preferred poly-sugars are for example cellulose and alkylderivatives thereof, preferably methyl cellulose, ethyl cellulose,propyl cellulose, butyl cellulose and their derivatives and mixtures ofat least two thereof. Preferred polymers which carry functional groupsoff of the main polymer chain are those which carry amide groups, thosewhich carry acid and/or ester groups, often called acrylic resins, orpolymers which carry a combination of aforementioned functional groups,or a combination thereof. Preferred polymers which carry amide off ofthe main chain are for example polyvinyl pyrrolidone (PVP) and itsderivatives. Preferred polymers which carry acid and/or ester groups offof the main chain are for example polyacrylic acid and its derivatives,polymethacrylate (PMA) and its derivatives or polymethylmethacrylate(PMMA) and its derivatives, or a mixture thereof. Preferred monomericbinders according to the present invention are ethylene glycol basedmonomers, terpineol resins or rosin derivatives, or a mixture thereof.Preferred monomeric binders based on ethylene glycol are those withether groups, ester groups or those with an ether group and an estergroup, preferred ether groups being methyl, ethyl, propyl, butyl, pentylhexyl and higher alkyl ethers, the preferred ester group being acetateand its alkyl derivatives, preferably ethylene glycol monobutylethermonoacetate or a mixture thereof. Alkyl cellulose, preferably ethylcellulose, its derivatives and mixtures thereof with other binders fromthe preceding lists of binders or otherwise are the most preferredbinders in the context of the present invention.

Surfactant

Preferred surfactants in the context of the present invention are thosewhich contribute to the formation of an electro-conductive paste withfavourable stability, printability, viscosity, sintering and etchingproperties. Surfactants are well known to the person skilled in the art.All surfactants which are known to the person skilled in the art andwhich he considers to be suitable in the context of this invention canbe employed as the surfactant in the organic vehicle. Preferredsurfactants in the context of the present invention are those based onlinear chains, branched chains, aromatic chains, fluorinated chains,siloxane chains, polyether chains and combinations thereof. Preferredsurfactants are single chained double chained or poly chained. Preferredsurfactants according to the present invention have non-ionic, anionic,cationic, or zwitterionic heads. Preferred surfactants are polymeric andmonomeric or a mixture thereof. Preferred surfactants according to thepresent invention can have pigment affinic groups, preferablyhydroxyfunctional carboxylic acid esters with pigment affinic groups(e.g., DISPERBYK®-108, manufactured by BYK USA, Inc.), acrylatecopolymers with pigment affinic groups (e.g., DISPERBYK®-116,manufactured by BYK USA, Inc.), modified polyethers with pigment affinicgroups (e.g., TEGO® DISPERS 655, manufactured by Evonik Tego ChemieGmbH), other surfactants with groups of high pigment affinity (e.g.,TEGO® DISPERS 662 C, manufactured by Evonik Tego Chemie GmbH). Otherpreferred polymers according to the present invention not in the abovelist are polyethyleneglycol and its derivatives, and alkyl carboxylicacids and their derivatives or salts, or mixtures thereof. The preferredpolyethyleneglycol derivative according to the present invention ispoly(ethyleneglycol)acetic acid. Preferred alkyl carboxylic acids arethose with fully saturated and those with singly or poly unsaturatedalkyl chains or mixtures thereof. Preferred carboxylic acids withsaturated alkyl chains are those with alkyl chains lengths in a rangefrom 8 to 20 carbon atoms, preferably C₉H₁₉COOH (capric acid),C₁₁H₂₃COOH (Lauric acid), C₁₃H₂₇COOH (myristic acid) C₁₅H₃₁COOH(palmitic acid), C₁₇H₃₅COOH (stearic acid) or mixtures thereof.Preferred carboxylic acids with unsaturated alkyl chains are C₁₈H₃₄O₂(oleic acid) and C₁₈H₃₂O₂ (linoleic acid). The preferred monomericsurfactant according to the present invention is benzotriazole and itsderivatives.

Solvent

Preferred solvents according to the present invention are constituentsof the electro-conductive paste which are removed from the paste to asignificant extent during firing, preferably those which are presentafter firing with an absolute weight reduced by at least 80% compared tobefore firing, preferably reduced by at least 95% compared to beforefiring. Preferred solvents according to the present invention are thosewhich allow an electro-conductive paste to be formed which hasfavourable viscosity, printability, stability and sinteringcharacteristics and which yields electrodes with favourable electricalconductivity and electrical contact to the substrate. Solvents are wellknown to the person skilled in the art. All solvents which are known tothe person skilled in the art and which he considers to be suitable inthe context of this invention can be employed as the solvent in theorganic vehicle. According to the present invention preferred solventsare those which allow the preferred high level of printability of theelectro-conductive paste as described above to be achieved. Preferredsolvents according to the present invention are those which exist as aliquid under standard ambient temperature and pressure (SATP) (298.15 K,25° C., 77° F.), 100 kPa (14.504 psi, 0.986 atm), preferably those witha boiling point above 90° C. and a melting point above −20° C. Preferredsolvents according to the present invention are polar or non-polar,protic or aprotic, aromatic or non-aromatic. Preferred solventsaccording to the present invention are mono-alcohols, di-alcohols,poly-alcohols, mono-esters, di-esters, poly-esters, mono-ethers,di-ethers, poly-ethers, solvents which comprise at least one or more ofthese categories of functional group, optionally comprising othercategories of functional group, preferably cyclic groups, aromaticgroups, unsaturated-bonds, alcohol groups with one or more O atomsreplaced by heteroatoms, ether groups with one or more O atoms replacedby heteroatoms, esters groups with one or more O atoms replaced byheteroatoms, and mixtures of two or more of the aforementioned solvents.Preferred esters in this context are di-alkyl esters of adipic acid,preferred alkyl constituents being methyl, ethyl, propyl, butyl, pentyl,hexyl and higher alkyl groups or combinations of two different suchalkyl groups, preferably dimethyladipate, and mixtures of two or moreadipate esters. Preferred ethers in this context are diethers,preferably dialkyl ethers of ethylene glycol, preferred alkylconstituents being methyl, ethyl, propyl, butyl, pentyl, hexyl andhigher alkyl groups or combinations of two different such alkyl groups,and mixtures of two diethers. Preferred alcohols in this context areprimary, secondary and tertiary alcohols, preferably tertiary alcohols,terpineol and its derivatives being preferred, or a mixture of two ormore alcohols. Preferred solvents which combine more than one differentfunctional groups are 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate,often called texanol, and its derivatives, 2-(2-ethoxyethoxy)ethanol,often known as carbitol, its alkyl derivatives, preferably methyl,ethyl, propyl, butyl, pentyl, and hexyl carbitol, preferably hexylcarbitol or butyl carbitol, and acetate derivatives thereof, preferablybutyl carbitol acetate, or mixtures of at least 2 of the aforementioned.

Additives in the Organic Vehicle

Preferred additives in the organic vehicle are those additives which aredistinct from the aforementioned vehicle components and which contributeto favourable properties of the electro-conductive paste, such asadvantageous viscosity, sintering, electrical conductivity of theproduced electrode and good electrical contact with substrates. Alladditives known to the person skilled in the art and which he considersto be suitable in the context of the present invention can be employedas additive in the organic vehicle. Preferred additives according to thepresent invention are thixotropic agents, viscosity regulators,stabilising agents, inorganic additives, thickeners, emulsifiers,dispersants or pH regulators. Preferred thixotropic agents in thiscontext are carboxylic acid derivatives, preferably fatty acidderivatives or combinations thereof. Preferred fatty acid derivativesare C₉H₁₉COOH (capric acid), C₁₁H₂₃COOH (Lauric acid), C₁₃H₂₇COOH(myristic acid) C₁₅H₃₁COOH (palmitic acid), C₁₇H₃₅COOH (stearic acid)C₁₈H₃₄O₂ (oleic acid), C₁₈H₃₂O₂ (linoleic acid) or combinations thereof.A preferred combination comprising fatty acids in this context is castoroil.

Additives in the Electro-Conductive Paste

Preferred additives in the context of the present invention areconstituents added to the electro-conductive paste, in addition to theother constituents explicitly mentioned, which contribute to increasedperformance of the electro-conductive paste, of the electrodes producedthereof or of the resulting solar cell. All additives known to theperson skilled in the art and which he considers suitable in the contextof the present invention can be employed as additive in theelectro-conductive paste. In addition to additives present in thevehicle, additives can also be present in the electro-conductive paste.Preferred additives according to the present invention are thixotropicagents, viscosity regulators, emulsifiers, stabilising agents or pHregulators, inorganic additives, thickeners and dispersants or acombination of at least two thereof, whereas inorganic additives aremost preferred. Preferred inorganic additives in this context accordingto the present invention are Mg, Ni, Te, W, Zn, Mg, Gd, Ce, Zr, Ti, Mn,Sn, Ru, Co, Fe, Cu and Cr or a combination of at least two thereof,preferably Zn, Sb, Mn, Ni, W, Te and Ru or a combination of at least twothereof, oxides thereof, compounds which can generate those metal oxideson firing, or a mixture of at least two of the aforementioned metals, amixture of at least two of the aforementioned oxides, a mixture of atleast two of the aforementioned compounds which can generate those metaloxides on firing, or mixtures of two or more of any of the abovementioned.

Process for Producing a Solar Cell

A contribution to achieving at one of the aforementioned objects is madeby a process for producing a solar cell at least comprising thefollowing as process steps:

-   -   i) provision of a solar cell precursor as described above, in        particular combining any of the above described embodiments; and    -   ii) firing of the solar cell precursor to obtain a solar cell.        Printing

It is preferred according to the present invention that the front andback electrodes are applied by applying an electro-conductive paste andthen firing said electro-conductive paste to obtain a sintered body. Theelectro-conductive paste can be applied in any manner known to theperson skilled in that art and which he considers suitable in thecontext of the present invention including but not limited toimpregnation, dipping, pouring, dripping on, injection, spraying, knifecoating, curtain coating, brushing or printing or a combination of atleast two thereof, wherein preferred printing techniques are ink-jetprinting, screen printing, tampon printing, offset printing, reliefprinting or stencil printing or a combination of at least two thereof.It is preferred according to the present invention that theelectro-conductive paste is applied by printing, preferably by screenprinting. It is preferred according to the present invention that thescreens have mesh opening with a diameter in a range from 20 to 100 μm,more preferably in a range from 30 to 80 μm, and most preferably in arange from 40 to 70 μm.

Firing

It is preferred according to the present invention for electrodes to beformed by first applying an electro-conductive paste and then firingsaid electro-conductive paste to yield a solid electrode body. Firing iswell known to the person skilled in the art and can be effected in anymanner known to him and which he considers suitable in the context ofthe present invention. Firing must be carried out above the glasstransition temperature of the glass frit.

According to the present invention the maximum temperature set for thefiring is below 900° C., preferably below 860° C. Firing temperatures aslow as 820° C. have been employed for obtaining solar cells. It ispreferred according to the present invention for firing to be carriedout in a fast firing process with a total firing time in the range from30 s to 3 minutes, more preferably in the range from 30 s to 2 minutesand most preferably in the range from 40 s to 1 minute. The time above600° C. is most preferably from 3 to 7 s.

Firing of electro-conductive pastes on the front and back faces can becarried out simultaneously or sequentially. Simultaneous firing isappropriate if the electro-conductive pastes applied to both faces havesimilar, preferably identical, optimum firing conditions. Whereappropriate, it is preferred according to the present invention forfiring to be carried out simultaneously. Where firing is affectedsequentially, it is preferable according to the present invention forthe back electro-conductive paste to be applied and fired first,followed by application and firing of the electro-conductive paste tothe front face.

Solar Cell

A contribution to achieving at least one of the above described objectsis made by a solar cell obtainable by a process according to the presentinvention. Preferred solar cells according to the present invention arethose which have a high efficiency in terms of proportion of totalenergy of incident light converted into electrical energy output andwhich are light and durable. The common configuration of a solar cellaccording to the present invention (excluding layers which are purelyfor chemical and mechanical protection) is as depicted in FIG. 2. Thelayer configuration shown there is given as follows: (i) Frontelectrode, (ii) Anti reflection coating, (iii) Front passivation layer,(iv) Front doped layer, (v) p-n junction boundary, (vi) Back dopedlayer, (vii) Highly doped back layer, (viii) Back passivation layer,(ix) Back electrode. Individual layers can be omitted from this commonlayer configuration or individual layers can indeed perform the functionof more than one of the layers described in the common embodimentoutlined above. In one embodiment of the present invention, a singlelayer acts as both anti-reflection layer and passivation layer. Theminimum required layer configuration is given in FIG. 1. This minimumlayer configuration is as follows: (I) Front electrode, (II) Front dopedlayer, (III) p-n junction boundary, (IV) Back doped layer, (V) Backelectrode.

In one embodiment of the invention, the solar cell comprises a waferwith a sheet resistance of at least 80 Ohm/sq., preferably at least 90Ohm/sq. more preferably at least 100 Ohm/sq. In some cases, a maximumvalue of 200 Ohm/sq. is observed for the sheet resistance of high Ohmicwafers.

Anti-Reflection Coating

According to the present invention, an anti-reflection coating can beapplied as the outer and often as the outermost layer before theelectrode on the front face of the solar cell. Preferred anti-reflectioncoatings according to the present invention are those which decrease theproportion of incident light reflected by the front face and increasethe proportion of incident light crossing the front face to be absorbedby the wafer. Anti-reflection coatings which give rise to a favourableabsorption/reflection ratio, are susceptible to etching by the employedelectro-conductive paste but are otherwise resistant to the temperaturesrequired for firing of the electro-conductive paste, and do notcontribute to increased recombination of electrons and holes in thevicinity of the electrode interface are favoured. All anti-reflectioncoatings known to the person skilled in the art and which he considersto be suitable in the context of the present invention can be employed.Preferred anti-reflection coatings according to the present inventionare SiN_(x), SiO₂, Al₂O₃, TiO₂ or mixtures of at least two thereofand/or combinations of at least two layers thereof, wherein SiN_(x) isparticularly preferred, in particular where an Si wafer is employed.

The thickness of anti-reflection coatings is suited to the wavelength ofthe appropriate light. According to the present invention it ispreferred for anti-reflection coatings to have a thickness in a rangefrom 20 to 300 nm, more preferably in a range from 40 to 200 nm and mostpreferably in a range from 60 to 90 nm.

Passivation Layers

According to the present invention, one or more passivation layers canbe applied to the front and/or back side as outer or as the outermostlayer before the electrode, or before the anti-reflection layer if oneis present. Preferred passivation layers are those which reduce the rateof electron/hole recombination in the vicinity of the electrodeinterface. Any passivation layer which is known to the person skilled inthe art and which he considers to be suitable in the context of thepresent invention can be employed. Preferred passivation layersaccording to the present invention are silicon nitride, silicon dioxideand titanium dioxide, silicon nitride being most preferred. According tothe present invention, it is preferred for the passivation layer to havea thickness in a range from 0.1 nm to 2 μm, more preferably in a rangefrom 10 nm to 1 μm and most preferably in a range from 30 nm to 200 nm.

Electrodes

A contribution to achieving at least one of the above mentioned objectsis made by a solar cell comprising at least one electrode with a halogenatom/ion content in a range from 0.1 to 40 mmol/kg, more preferably in arange from 1 to 30 mmol/kg, and most preferably in a range from 10 to 20mmol/kg, in each case based on the number of halogen atoms/ions and onthe total weight of the electrode.

Additional Protective Layers

In addition to the layers described above which directly contribute tothe principle function of the solar cell, further layers can be addedfor mechanical and chemical protection. The cell can be encapsulated toprovide chemical protection. Encapsulations are well known to the personskilled in the art and any encapsulation can be employed which is knownto him and which he considers suitable in the context of the presentinvention. According to the present invention, transparent polymers,often referred to as transparent thermoplastic resins, are preferred asthe encapsulation material, if such an encapsulation is present.Preferred transparent polymers in this context are for example siliconrubber and polyethylene vinyl acetate (PVA).

A transparent glass sheet can be added to the front of the solar cell toprovide mechanical protection to the front face of the cell. Transparentglass sheets are well known to the person skilled in the art and anytransparent glass sheet known to him and which he considers to besuitable in the context of the present invention can be employed asprotection on the front face of the solar cell.

A back protecting material can be added to the back face of the solarcell to provide mechanical protection. Back protecting materials arewell known to the person skilled in the art and any back protectingmaterial which is known to the person skilled in the art and which heconsiders to be suitable in the context of the present invention can beemployed as protection on the back face of the solar cell. Preferredback protecting materials according to the present invention are thosehaving good mechanical properties and weather resistance. The preferredback protection materials according to the present invention ispolyethylene terephthalate with a layer of polyvinyl fluoride. It ispreferred according to the present invention for the back protectingmaterial to be present underneath the encapsulation layer (in the eventthat both a back protection layer and encapsulation are present).

A frame material can be added to the outside of the solar cell to givemechanical support. Frame materials are well known to the person skilledin the art and any frame material known to the person skilled in the artand which he considers suitable in the context of the present inventioncan be employed as frame material. The preferred frame materialaccording to the present invention is aluminium.

Solar Panels

A contribution to achieving at least one of the above mentioned objectsis made by a module comprising at least a solar cell obtained asdescribed above, in particular according to at least one of the abovedescribed embodiments, and at least one more solar cell. A multiplicityof solar cells according to the present invention can be arrangedspatially and electrically connected to form a collective arrangementcalled a module. Preferred modules according to the present inventioncan take a number of forms, preferably a rectangular surface known as asolar panel. A large variety of ways to electrically connect solar cellsas well as a large variety of ways to mechanically arrange and fix suchcells to form collective arrangements are well known to the personskilled in the art and any such methods known to him and which heconsiders suitable in the context of the present invention can beemployed. Preferred methods according to the present invention are thosewhich result in a low mass to power output ratio, low volume to poweroutput ration, and high durability. Aluminium is the preferred materialfor mechanical fixing of solar cells according to the present invention.

DESCRIPTION OF THE DRAWINGS

The present invention is now explained by means of figures which areintended for illustration only and are not to be considered as limitingthe scope of the present invention. In brief,

FIG. 1 shows a cross sectional view of the minimum layer configurationfor a solar cell,

FIG. 2 shows a cross sectional view a common layer configuration for asolar cell,

FIGS. 3a, 3b and 3c together illustrate the process of firing a frontside paste.

FIG. 4 shows the positioning of cuts for the test method below tomeasure specific contact resistance.

FIG. 1 shows a cross sectional view of a solar cell 100 and representsthe minimum required layer configuration for a solar cell according tothe present invention. Starting from the back face and continuingtowards the front face the solar cell 100 comprises a back electrode104, a back doped layer 106, a p-n junction boundary 102, a front dopedlayer 105 and a front electrode 103, wherein the front electrodepenetrates into the front doped layer 105 enough to form a goodelectrical contact with it, but not so much as to shunt the p-n junctionboundary 102. The back doped layer 106 and the front doped layer 105together constitute a single doped Si wafer 101. In the case that 100represents an n-type cell, the back electrode 104 is preferably a silverelectrode, the back doped layer 106 is preferably Si lightly doped withP, the front doped layer 105 is preferably Si heavily doped with B andthe front electrode 103 is preferably a mixed silver and aluminiumelectrode. In the case that 100 represents a p-type cell, the backelectrode 104 is preferably a mixed silver and aluminium electrode, theback doped layer 106 is preferably Si lightly doped with B, the frontdoped layer 105 is preferably Si heavily doped with P and the frontelectrode 103 is preferably a silver electrode. The front electrode 103has been represented in FIG. 1 as consisting of three bodies purely toillustrate schematically the fact that the front electrode 103 does notcover the front face in its entirety. The present invention does notlimit the front electrode 103 to those consisting of three bodies.

FIG. 2 shows a cross sectional view of a common layer configuration fora solar cell 200 according to the present invention (excludingadditional layers which serve purely for chemical and mechanicalprotection). Starting from the back face and continuing towards thefront face the solar cell 200 comprises a back electrode 104, a backpassivation layer 208, a highly doped back layer 210, a back doped layer106, a p-n junction boundary 102, a front doped layer 105, a frontpassivation layer 207, an anti-reflection layer 209, front electrodefingers 214 and front electrode bus bars 215, wherein the frontelectrode fingers penetrate through the anti-reflection layer 209 andthe front passivation layer 207 and into the front doped layer 105 farenough to form a good electrical contact with the front doped layer, butnot so far as to shunt the p-n junction boundary 102. In the case that200 represents an n-type cell, the back electrode 104 is preferably asilver electrode, the highly doped back layer 210 is preferably Siheavily doped with P, the back doped layer 106 is preferably Si lightlydoped with P, the front doped layer 105 is preferably Si heavily dopedwith B, the anti-reflection layer 209 is preferably a layer of siliconnitride and the front electrode fingers and bus bars 214 and 215 arepreferably a mixture of silver and aluminium. In the case that 200represents a p-type cell, the back electrode 104 is preferably a mixedsilver and aluminium electrode, the highly doped back layer 210 ispreferably Si heavily doped with B, the back doped layer 106 ispreferably Si lightly doped with B, the front doped layer 105 ispreferably Si heavily doped with P, the anti-reflection layer 209 ispreferably a layer of silicon nitride and the front electrode fingersand bus bars 214 and 215 are preferably silver. FIG. 2 is schematic andthe invention does not limit the number of front electrode fingers tothree as shown. This cross sectional view is unable to effectively showthe multitude of front electrode bus bars 215 arranged in parallel linesperpendicular to the front electrode fingers 214.

FIGS. 3a, 3b and 3c together illustrate the process of firing a frontside paste to yield a front side electrode. FIGS. 3a, 3b and 3c areschematic and generalised and additional layers further to thoseconstituting the p-n junction are considered simply as optionaladditional layers without more detailed consideration.

FIG. 3a illustrates a wafer before application of front electrode, 300a. Starting from the back face and continuing towards the front face thewafer before application of front electrode 300 a optionally comprisesadditional layers on the back face 311, a back doped layer 106, a p-njunction boundary 102, a front doped layer 105 and additional layers onthe front face 312. The additional layers on the back face 311 cancomprise any of a back electrode, a back passivation layer, a highlydoped back layer or none of the above. The additional layer on the frontface 312 can comprise any of a front passivation layer, ananti-reflection layer or none of the above.

FIG. 3b shows a wafer with electro-conductive paste applied to the frontface before firing 300 b. In addition to the layers present in 300 adescribed above, an electro-conductive paste 313 is present on thesurface of the front face.

FIG. 3c shows a wafer with front electrode applied 300 c. In addition tothe layers present in 300 a described above, a front side electrode 103is present which penetrates from the surface of the front face throughthe additional front layers 312 and into the front doped layer 105 andis formed from the electro-conductive paste 313 of FIG. 3b by firing.

In FIGS. 3b and 3c , the applied electro-conductive paste 313 and thefront electrodes 103 are shown schematically as being present as threebodies. This is purely a schematic way of representing a non-completecoverage of the front face by the paste/electrodes and the presentinvention does not limit the paste/electrodes to being present as threebodies.

FIG. 4 shows the positioning of cuts 421 relative to finger lines 422 inthe wafer 420 for the test method below to measure specific contactresistance.

Test Methods

The following test methods are used in the present invention. In absenceof a test method, the ISO test method for the feature to be measuredbeing closest to the earliest filing date of the present applicationapplies. In absence of distinct measuring conditions, standard ambienttemperature and pressure (SATP) as a temperature of 298.15 K (25° C.,77° F.) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm)apply.

Viscosity

Viscosity measurements were performed using the Thermo FischerScientific Corp. “Haake Rheostress 600” equipped with a ground plateMPC60 Ti and a cone plate C 20/0.5° Ti and software “Haake RheoWin JobManager 4.30.0”. After setting the distance zero point, a paste samplesufficient for the measurement was placed on the ground plate. The conewas moved into the measurement positions with a gap distance of 0.026 mmand excess material was removed using a spatula. The sample wasequilibrated to 25° C. for three minutes and the rotational measurementstarted. The shear rate was increased from 0 to 20 s⁻¹ within 48 s and50 equidistant measuring points and further increased to 150 s⁻¹ within312 s and 156 equidistant measuring points. After a waiting time of 60 sat a shear rate of 150 s⁻¹, the shear rate was reduced from 150 s⁻¹ to20 s⁻¹ within 312 s and 156 equidistant measuring points and furtherreduced to 0 within 48 s and 50 equidistant measuring points. The microtorque correction, micro stress control and mass inertia correction wereactivated. The viscosity is given as the measured value at a shear rateof 100 s⁻¹ of the downward shear ramp.

Specific Contact Resistance

In an air conditioned room with a temperature of 22±1° C., all equipmentand materials are equilibrated before the measurement. For measuring thespecific contact resistance of fired silver electrodes on the frontdoped layer of a silicon solar cell a “GP4-Test Pro” equipped with the“GP-4 Test 1.6.6 Pro” software package from the company GP solar GmbH isused. This device applies the 4 point measuring principle and estimatesthe specific contact resistance by the transfer length method (TLM). Tomeasure the specific contact resistance, two 1 cm wide stripes of thewafer are cut perpendicular to the printed finger lines of the wafer asshown in FIG. 4. The exact width of each stripe is measured by amicrometer with a precision of 0.05 mm. The width of the fired silverfingers is measured on 3 different spots on the stripe with a digitalmicroscope “VHX-600D” equipped with a wide-range zoom lens VH-Z100R fromthe company Keyence Corp. On each spot, the width is determined tentimes by a 2-point measurement. The finger width value is the average ofall 30 measurements. The finger width, the stripe width and the distanceof the printed fingers to each other is used by the software package tocalculate the specific contact resistance. The measuring current is setto 14 mA. A multi contact measuring head (part no. 04.01.0016) suitableto contact 6 neighboring finger lines is installed and brought intocontact with 6 neighboring fingers. The measurement is performed on 5spots equally distributed on each stripe. After starting themeasurement, the software determines the value of the specific contactresistance (mOhm*cm²) for each spot on the stripes. The average of allten spots is taken as the value for specific contact resistance.

Sheet Resistance

For measuring the sheet resistance of a doped silicon wafer surface, thedevice “GP4-Test Pro” equipped with software package “GP-4 Test 1.6.6Pro” from the company GP solar GmbH is used. For the measurement, the 4point measuring principle is applied. The two outer probes apply aconstant current and two inner probes measure the voltage. The sheetresistance is deduced using the Ohmic law in Ohm/square. To determinethe average sheet resistance, the measurement is performed on 25 equallydistributed spots of the wafer. In an air conditioned room with atemperature of 22±1° C., all equipment and materials are equilibratedbefore the measurement. To perform the measurement, the “GP-Test.Pro” isequipped with a 4-point measuring head (part no. 04.01.0018) with sharptips in order to penetrate the anti-reflection and/or passivationlayers. A current of 10 mA is applied. The measuring head is broughtinto contact with the non metalized wafer material and the measurementis started. After measuring 25 equally distributed spots on the wafer,the average sheet resistance is calculated in Ohm/square.

Particle Size

A typically method to determine d₁₀, d₉₀ and d₅₀ is for exampledescribed in DIN EN 725-5.

Glass Transition Temperature (T_(g))

T_(g) is determined by Differential Scanning calorimetry DSC (measuringheat capacity).

Dopant Level

Dopant levels are measured using secondary ion mass spectroscopy.

EXAMPLES

The present invention is now explained by means of examples which areintended for illustration only and are not to be considered as limitingthe scope of the present invention.

Example 1—Paste Preparation

A paste was made by mixing the appropriate amounts of organic vehicle(Table 1), Ag powder (PV 4 from Ames Inc. with a d₅₀ of 2 μm), highlead-content borosilicate glass frit ground to d₅₀ of 2 μm (F-010 fromHeraeus Precious Metals GmbH & Co. KG.) and a halogen containingcompound according to the specific example. The paste was passed througha 3-roll mill at progressively increasing pressures from 0 to 8 bar. Theviscosity was measured as mentioned above and appropriate amounts oforganic vehicle with the composition given in Table 1 were added toadjust the paste viscosity toward a target of between 16-20 Pas. The wt.% s of the constituents of the paste are given in Table 2. The ZnOpowder was obtained from Sigma Aldrich GmbH (article number 204951).

TABLE 1 Wt. % based on total Organic Vehicle Component weight of OrganicVehicle 2-(2-butoxyethoxy)ethanol) [solvent] 84 ethyl cellulose (DOWEthocel 4) [binder] 6 Thixcin ® E [thixotropic agent] 10

TABLE 2 Halogen Wt. % of Wt. % of Containing Wt. % of Wt. % of Wt % ofOrganic halogen containing Example # compound Ag powder Glass Frit ZnOVehicle compound 1 AgCl 84.98 3.5 0.5 10.8 0.22 2 ZnCl2 85.08 3.5 0.510.8 0.12 3 PbCl2 84.99 3.5 0.5 10.8 0.21 4 InCl3 85.08 3.5 0.5 10.80.12 5 LiF 85.08 3.5 0.5 10.8 0.12 Comparative — 85 3.5 0.5 11 0

Example 2—Solar Cell Preparation and Contact Resistance Measurement

Pastes were applied to fullsquare mono-crystalline p-type wafers with aback doped layer resistivity in the range from 0.1 to 10 Ohm*cm and witha lightly doped n-type emitter (LDE) with a surface doping concentrationof 2*10²⁰ cm⁻³ and a sheet resistance of 90 Ohm/square. The waferdimensions were 156×156 mm, the front side had a textured surfaceapplied by an alkaline etching process. The front side was also coatedwith a 70 nm thick PECVD (plasma enhanced chemical vapour deposition)SiN_(x) passivation and anti-reflective layer, commercially availablefrom Fraunhofer ISE. The example paste was screen-printed onto theilluminated (front) face of the wafer using a ASYSAutomatisierungssysteme GmbH Ekra E2 screen printer and a standardH-pattern screen from Koenen GmbH. The screen had 75 finger lines with80 μm openings and three 1.5 mm wide Busbars. The Emulsion over mesh wasin the range from 16 to 20 μm, the screen had 300 mesh and 20 μmstainless steel wire. The printing parameters were 1.2 bar squeegeepressure, forward squeegee speed 150 mm/s and flooding speed 200 mm/s Acommercially available Al paste, Gigasolar 108 from Giga Solar MaterialsCorp., was printed on the non-illuminated (back) face of the device. Thedevice with the printed patterns on both sides was then dried in an ovenfor 10 minutes at 150° C. The substrates were then fired sun-side upwith a Centrotherm Cell & Module GmbH c-fire fast firing furnace. Thefurnace consists of 6 zones. Zone 1 was set to 350° C., zone 2 to 475°C., zone 3 to 470° C., zone 4 to 540° C., zone 5 to 840° C. and zone 6to 880° C. The belt speed was set to 5100 mm/min. The fully processedsamples were then tested for contact resistance using the abovementioned method, shown in Table 3. For each paste, the normalisedvalues of the contact resistance for 6 samples are shown.

TABLE 3 Concentration of Halogen Halogen containing Atoms NormalisedExample # compound mmol/kg Contact Resistance 1 AgCl 15 25% 2 PbCl₂ 1530% 3 ZnCl₂ 17 90% 4 InCl₃ 16 50% 5 LiF 46 344% comparative none 0 100%

REFERENCE LIST

-   101 doped Si wafer-   102 p-n junction boundary-   103 Front electrode-   104 Back electrode-   105 Front doped layer-   106 Back doped layer-   207 Front passivation layer-   208 Back passivation layer-   209 Anti-reflection layer-   210 Highly doped back layer-   311 Additional layers on back face-   312 Additional layers on front face-   313 Electro-conductive paste-   214 Front electrode fingers-   215 Front electrode bus bars-   420 Wafer-   421 Cuts-   422 Finger lines

The invention claimed is:
 1. A solar cell precursor comprising asprecursor parts: i) a wafer with sheet resistance of at least 80Ohm/square; ii) an electro-conductive paste comprising: a) metallicparticles; b) a glass frit; c) an organic vehicle; and d) a halogencontaining compound applied to the wafer; wherein the halogen containingcompound comprises a ClO₄ ⁻ ion, and the halogen containing compound ispresent in the paste in a range from 10 to 40 mmol/kg based on thenumber of halogen atoms/ions and the total weight of the paste.
 2. Thesolar cell precursor according to claim 1, wherein the halogencontaining compound is present in the paste in a range from 10 to 30mmol/kg based on the number of halogen atoms/ions and the total weightof the paste.
 3. The solar cell precursor according to claim 2, whereinthe halogen containing compound has a melting point in a range from 180to 800° C.
 4. The solar cell precursor according to claim 1, wherein theglass frit is present in the paste as less than 4 wt. %.
 5. The solarcell precursor according to claim 4, wherein the halogen containingcompound has a melting point in a range from 180 to 800° C.
 6. The solarcell precursor according to claim 1, wherein the halogen containingcompound has a melting point in a range from 180 to 800° C.
 7. The solarcell precursor according to claim 1, wherein the metallic particlescomprise Ag.
 8. The solar cell precursor according to claim 1, whereinthe electro-conductive paste is on the front face of the wafer.
 9. Thesolar cell precursor according to claim 1, wherein the wafer has a sheetresistance of at least 90 Ohm/square.
 10. The solar cell precursoraccording to claim 1, wherein the halogen containing compound is presentin the paste in a range from 10 to 20 mmol/kg based on the number ofhalogen atoms/ions and the total weight of the paste.
 11. The solar cellprecursor according to claim 1, wherein the halogen containing compoundcomprises a counter-ion selected from Ag⁺ and Zn²⁺.
 12. The solar cellprecursor according to claim 1, wherein the halogen containing compoundcomprises Ag⁺ as a counter-ion.
 13. A process for the preparation of asolar cell comprising the steps: i) providing a solar cell precursoraccording to claim 1; ii) firing the solar cell precursor to obtain asolar cell.
 14. The process according to claim 13, wherein the maximumtemperature during the firing step is less than 860° C.
 15. A solar cellobtained by the process according to claim
 13. 16. A module comprisingat least one solar cell according to claim 15 and at least a furthersolar cell.